HEART-URCHINS. 193 



perforated portions of the ambulacra! areas are now 

 restricted merely to the central region of the upper 

 surface, one of these areas (directed upwards in the figure) 

 forming a shallower groove, and being otherwise markedly 

 different from all the others. In such an urchin the 

 bilateral symmetry is very strongly marked indeed, and, 

 since the upper border of the figure represents the 

 anterior, and the lower the posterior extremity of the 

 animal, we may compare the three anterior ambulacral 

 areas to the head and arms of a quadruped whose hind 

 legs will be represented by the two posterior ambulacra. 

 In regard to the position of the two orifices of the test in 

 this species, the vent is situated on the flattened posterior 

 surface, near its junction with the upper surface, while the 

 mouth occupies a position nearly midway between the 

 centre and the anterior border of the lower surface, at the 

 commencement of the groove formed by the anterior 

 ambulacral area. The mouth is peculiar in that it does 

 not open directly on the surface, in the usual manner, but 

 has a projecting lip by means of which its aperture 

 assumes a forward direction. The common purple heart- 

 urchin (Spatangus] of our present seas is a larger 

 representative of this group, presenting the same general 

 type of structure. 



We thus see how gradual is the passage from a species 

 of the type of the common urchin to that of the heart- 

 urchin, widely different as are these two from one another. 

 We might, indeed, proceed further with our investigations, 

 and show how certain of the irregular urchins have become 

 so flattened as to assume the form of thin plates, which 

 in some instances are deeply notched at their periphery. 

 And we might also investigate the variation of form and 

 size displayed by the spines of the different groups. 

 Enough has, however, been written for our present object, 

 which has been to show the amount of interest that 

 attaches to the investigation of the lines of modification 

 on which development has proceeded among the sea-urchins. 

 This has shown how a regular progressive advance in one 

 particular direction has taken place from the earlier to the 

 later members of the group ; and we thus have another 



